Television transmission system



Jan. 10, 1939. E. L. c. WHITE 2,143,393

' TELEVISION TRANSMISSION SYSTEM 7 Filed June 9, 1936 2 Sheets-Sheet lAllll AAAAII v1 vv 74 1 11 1/ 9 JJ 12 18 .I T

=1? INVENTOR E. L. C. WHITE ATTORNEY Jam. 1Q, 1939. E, 3 WHITE 2,143,398

TELEVIS ION TRANSMI SS ION SYSTEM Filed June 9, 1936 2 Sheets-Sheet 2lnfie mafia 651E Rag 27 5mm) T0 with 45 g6 mg; a

Patented Jan. 10, 1939 UNITED STATES TELEVISION TRANSMISSION SYSTEM EricLawrence Casling White, Hillingdon, England, assignor to Electric &Musical Industries,

Ltd.

Application June 9,

1936, Serial No. 84,241

In Great Britain June 24, 1935 7 Claims.

The present invention relates to television, and is concerned moreparticularly with television transmission systems embodying cathode raytubes such for example as Iconoscopes.

An Iconoscope is a cathode ray tube having a mosaic screen comprising amultiplicity of photo-electrically sensitive elements insulated from oneanother and from a common signal plate; an image to be transmitted isformed on the photoelectric elements and means are provided for scanningthe elements periodically with the oathode ray of the tube to bring thepotentials of the elements to a datum value. Picture signals are usuallyderived from a signal resistance associated with the signal plate.Scanning of the screen is usually carried out in a series of parallellines, the pattern traced out by the scanning beam usually beingperiodically retraced at a frequency which will be referred to as theframe frequency.

It is found that in transmission systems employing an Iconoscope, thepicture signals developed include spurious components, there beingusually one such component at line frequency and one at frame frequency.The effect of such spurious signal components at a receiver, assumingthat the scanning takes place from left to right in horizontal linesproceeding from top to bottom, is usually found to be that the averagebrightness is greater on the right than on the left and greater at thebottom than at the top.

Similar spurious signals arise also in systems employing mosaic-screentransmitting tubes other than Iconoscopes, such for example as certaintubes embodying mosaic screens which are not photo-electricallysensitive. In such tubes, an electrostatic image is formed on. the,mosaic screen, in a manner such as is described in British Patent No.442,666, and the mosaic screen is scanned by the cathode ray tube as inan Iconoscope. The generation of the spurious signals is believed to bedue to the fact that as each element of the mosaic screen is scanned,secondary electrons are emitted therefrom; some of these secondaryelectrons flow to a separate collecting electrode, which may form partof the electron gun, but others are collected by neighbouring elementsof the screen, these latter electrons giving rise to the spurioussignals.

It has been proposed, in an attempt to compensate for spurious signalsof the kind discussed, to superimpose rectilinear saw tooth waves ofline and frame frequencies, and of suitable amplitude and sense, uponthe picture signals, with the object of cancelling out the spurioussignals and rendering the base line thereof substantially straight.

It has been found however, that only partial correction can be obtainedin this way, and it is the object of the present invention to providemeans whereby more complete cancellation of spurious signals of thecharacter discussed can be obtained.

According to the present invention, in television transmitting apparatusin which a cathode ray tube having a mosaic screen is employed todevelop picture signals, the screen being scanned in a plurality ofstraight, parallel lines, and in which the arrangement is such that thepicture signals developed include spurious components of the characterset forth, there are provided means for developing a corrective signalcomprising an oscillation having a wave form which is substantiallyproportional to an inte gral of a rectilinear saw tooth wave, and meansfor superimposing said corrective signal upon the picture signal derivedfrom said tube so that the spurious components of said picture signalsare eliminated or reduced. Preferably two such corrective signals, oneof line frequency and one of frame frequency are superimposed upon thepicture signals. The wave form of the corrective signal may comprise acomponent proportional to the simple integral or to some higher integralof a rectilinear saw tooth wave or the corrective signal may comprise aplurality of components proportional to different integrals.

In the following further description of the invention, reference will bemade to the accompanying drawings, in which Figure 1 illustrates oneform of integrating circuit.

Figure 2 represents another form of integrating circuit.

Figure 3 represents still another form.

Figure 4 is an embodiment for producing integrated oscillations.

Figure 5 is another embodiment for producing integrated oscillations,and

Fig. 6 shows a mixing of the integrated oscillations and the picturesignals containing spurious oscillations.

The invention will be described, by way of example, as applied to atelevision transmission system employing an Iconoscope, the cathode raybeing deflected for scanning purposes by means of two sets of deflectingcoils through which, respectively, saw tooth currents of line and framefrequency are passed; it will be assumed that the scanning oscillationsare generated by tWo blocking oscillators, one operating at linefrequency and the other at frame frequency, the saw tooth oscillationsbeing fed to separate power output valves having the line and framedeflecting coils arranged in their respective anode circuits.

For the purpose of developing line frequency corrective oscillations,saw tooth oscillations derived from the line frequency blockingoscillator are fed to an integrating circuit of the kind illustrated inFig. 1, which comprises a resistance R in series with a condenser C,integrated oscillations being taken off from the terminals of thecondenser to the circuit in which the picture signals are present. Thetime constant of the condenser C and resistance R is made long comparedwith the line period. The integrated wave form then consists of twoparabolic arcs for each cycle of the saw tooth oscillation, a major arccorresponding to the less steeply inclined portion. of the saw toothwave and a minor arc corresponding to the more steeply inclined portion.The integrated waveform, which if derived may be mixed with a saw toothoscillation derived from the line frequency blocking oscillator, isapplied to the picture signal circuit in appropriate sense and at asuitable amplitude. The amplitude is adjusted, for example with the aidof a potentiometer, not shown, so that the spurious signal of linefrequency is substantially compensated as illustrated in Fig. 6 whichshows the standard mixing tube arrangement by means of which theintegrated compensating signal may be mixed in opposing phaserelationship to the spurious signal of line frequency. The amplitudewill depend naturally upon the intensity of the spurious signal asshown, for instance, by a monitor tube at the transmitter. The minor arcof the integrated wave form occurs during the return stroke of thescanning cathode ray, that is, during a time when no picture signals arebeing generated. The corrective signal is preferably injected into thepicture signal circuit at a point where the picture, signals haveundergone one or more stages of amplification.

A frame frequency corrective oscillation can be derived similarly byfeeding frame frequency saw tooth oscillations from the frame frequencyblocking oscillator to an integrating circuit of the same kind as thatabove described, but having a time constant long compared with the frameperiod.

In some cases it may be found that more accurate correction can beobtained by using a corrective wave including, in addition to a sawtooth oscillation and the first integral thereof, a higher integral ofthe saw tooth wave.

The arrangement show in Fig. 2 provides at its output terminals awavewhich is the second integral of a saw tooth oscillation applied atits input terminals I, if this Wave is mixed with the original saw toothoscillation and the first integral thereof, in suitable sense, a waveform comprising a saw tooth plus an approximate sine wave can beobtained. In Fig. 2, the time constants RC and RC' are both longcompared with the period of the applied saw tooth oscillation.

Each shunt arm of the networks of Figs. 1 and 2 may comprise aresistance, which is preferably made variable; in this case, the outputof the network includes a component of the wave form of the applied sawtooth oscillation, in addition to the integrated wave.

The circuit for developing a corrective signal of either sense which isillustrated in Fig. 3, comprises a resistance R connected between twoinput terminals I1 and I2, a variable tapping point on the resistance Rbeing connected through a resistance R and a condenser C to earth.Rectilinear saw tooth oscillations are fed in opposite senses to theterminals I1 and I2 respectively from a circuit, such as the outputcircuit of a push-pull amplifier, which is balanced with respect toearth, and the amplitude and sense of the corrective signals developedacross the condenser C are varied by moving the adjustable contact alongthe resistance R. In the arrangement of Fig. 3, the corrective signalset up at output terminal 0 with respect to earth comprises a saw toothcomponent and a parabolic component.

Referring now to Fig. 4, which shows a practical circuit arrangementaccording to the invention, a rectilinear saw tooth oscillation is fedto the control grid of a valve 6, such as a pentode, which has a highimpedance, through a condenser I. The anode of valve 6 is connected tothe positive terminal of a source (not shown) of anode current through aresistance 8, and the cathode thereof is connected to earth throughresistances 9 and ID in series; the negative terminal of the anodecurrent source is earthed, and a grid leak resistance II is connected asshown to the join of resistances 9 and I0.

A condenser I2 is connected between the anode of valve 6 and earth, andthe time constant of resistance 8 and condenser 9 is made long comparedwith the period of the applied saw tooth oscillation. Part of the anodecurrent of valve 6 flows in resistance 8 and part is integrated incondenser I2, the voltage set up across the condenser I2 being thus ofparabolic wave form.

Potential differences set up across condenser !2 are fed throughcondenser I4 to the control grid of valve I3, the anode of which isconnected to the positive terminal of the anode current source throughresistance I5, and the cathode of which is connected to earth throughtwo resistances I6 and I! in series. A grid leak I8 is connected betweenthe join of resistances I6 and I I and the grid of valve I3. The desiredintegrated wave form is taken oiT from the anode or cathode of valve l3,through condenser I9 or 28 respectively, depending upon the senserequired.

In the arrangement of Fig. 5, parts which are common to Fig. 4 have thesame references; a resistance 2| is arranged in series between the upperend of resistance 8 and the anode current source, and the total value ofresistance I6 and I I in series is made large compared to the reciprocalof the mutual conductance of valve I3 so that the potential ofv thecathode of that valve substantially follows variations in the gridpotential thereof; in other words, valve I3 operates as a cathodefollower valve. A condenser 22 is connected between the cathode of valveI3 and the join of resistances 8 and 2!.

In operation, the potential at the join of resistances 8 and 2Isubstantially follows variations in the potential of the control grid ofvalve I3 and hence of the anode of valve 6; thus a greater proportion ofthe anode current of valve 6 is integrated in condenser I2 than in thearrangement of Fig. 4, since resistance 8 is effectively much greater.In other words, by the use of valve I3 in the manner described as acathode follower, the efiective time constant of elements 8 and I2 isgreatly increased, and the voltage set up across condenser I2 is a moreaccurate integration of the applied saw tooth oscillation.

The resistance 8 may be regarded as a shunt leakage path acrosscondenser I 2, the resistance 8 having the effect of reducing theproportion of the anode current of valve 6 which is integrated in thecondenser I2. The feed back from valve I3 has the effect of increasingthe effective resistance of the leakage path, and it is pointed out thatthis method'of reducing effective shunt conductance, and consequenceleakage, is not limited in its application to integrating circuits, butis applicable in many other circuits and for many other purposes.

Referring to Fig. 6, there is shown the mixing of the integratedoscillations and the picture signals containing the spuriousoscillations as hereinbefore disclosed. A cathode ray transmitting tubeor so-called Iconoscope 40 has means 4| for developing potentials inaccordance with the light values of optical images electrically scanned.As hereinbefore shown, these signals contain spurious oscillations ofline frequency. The combined video and spurious signals are fed to anamplifier 42 and thence to a well known form of mixing tube arrangementshown as 43. The control oscillation or saw-tooth generator 44 developsa wave, a portion of which is fed to one of the hereinbefore disclosedintegrating circuits 45. The integrated wave may then be fed to a singleor plural stage amplifier 46, and the output of this amplifier is fed toa potentiometer 41. This potentiometer is joined to a resistance 48 inthe screen grid circuit of the mixing tube 43. The combined video andspurious signals are fed to the control grid 49 of mixing tube 43 and itwill be seen that the output current is con trolled both by the Waveshape of the waves impressed on both the control grid 49 and the screengrid 50 of the tube 43. The amplitude of the integrated signal impressedon the screen grid 50 is controlled from the potentiometer 41, and thoseskilled in the art will immediately recognize that this integrated waveshould be fed in phase opposition to the spurious signal of linefrequency occurring in the video signal, thus substantially or entirelyeliminating this undesired oscillation.

The invention is not limited to the arrangements described by way ofexample above, and many modifications within the scope of the appendedclaims will occur to those versed in the art.

What I claim is:

1. In cathode ray apparatus in which a cathode ray beam is deflected inaccordance with electrical oscillations of a definite wave form andelectrical impulses are sequentially derived from the apparatus due tothe action of the cathode ray, the method of correcting for spuriousoscillations occurring due to the deflected wave which comprises thesteps of developing a wave form which is an integral of the deflectingwave formation and combining the integrated Wave form with theaforementioned derived electrical impulses in phase opposition to thespurious wave forms occurring therein.

2. In cathode ray apparatus in which a oathode ray beam is deflected inaccordance with electrical oscillations of a definite wave form andelectrical impulses are sequentially derived from the apparatus due tothe action of the cathode ray, the method of correcting for spuriousoscillations occurring due to the deflected wave which comprises thesteps of developing the wave form which is an integral of the deflectingwave form, combining the integrated wave form and the deflecting waveform and then combining the wave form of this combination with thederived electrical impulses in phase opposition to the spurious waveform occurring therein.

3. In cathode ray apparatus in which a cathode ray beam is deflected inaccordance with electrical oscillations of a definite wave form andelectrical impulses are sequentially derived from the apparatus due tothe action of the cathode ray, the method of correcting for spuriousoscillations occurring due to the deflected wave which comprises thesteps of developing a wave form which consists of the first and higherintegrals of the deflecting wave form and then combining the wave formof this combination with the derived electrical impulses in phaseopposition to the spurious wave forms occurring therein.

4. A television transmission apparatus comprising a cathode ray tubecontaining a mosaic adapted to be scanned by the cathode ray, means fordeflecting the cathode ray in accordance with a definite wave formation,means for deriving electrical picture impulses from the mosaic as thescanning operation is performed, means for deriving from the deflectingwave formation a wave form which is the integral thereof, and means forcombining the integrated wave form with the picture signals in a phaserelationship so as to substantially erase spurious oscillation wavesincluded in the picture signals.

5. A television transmission apparatus comprising a cathode ray tubecontaining a mosaic adapted to be scanned by the cathode ray, means fordeflecting the cathode ray in accordance with a definite wave formation,means for deriving electrical picture impulses from the mosaic as thescanning operation is performed, means for deriving from the deflectingwave formation a wave form which is a combination of the first andhigher integrals of the deflecting wave form, and means for combiningthe integrated wave form with the picture signals in a phaserelationship so as to substantially erase spurious oscillation wavesincluded in the picture signals.

6. A television transmission apparatus comprising a cathode ray tubecontaining a mosaic adapted to be scanned by the cathode ray, means fordeflecting the cathode ray in accordance with a definite wave formation,means for derivin electrical picture impulses from the mosaic as thescanning operation is performed, means for deriving from the deflectingwave formation a wave form which is the integral thereof, means forcombining the integrated wave form with the deflecting wave form, andmeans for combining the resultant of the aforesaid combination of waveswith the picture signals in a phase relationship so as to substantiallyerase spurious oscillation waves contained in the picture signals.

7. A television transmission apparatus comprising a cathode ray tubecontaining a mosaic adapted to be scanned by the cathode ray, means fordeflecting the cathode ray in accordance with a definite wave formation,means for deriving electrical picture impulses from the mosaic as thescanning operation is performed, means for deriving from the deflectingwave formation a wave form which is a combination of the first andhigher integrals of the deflecting wave form, means for combining theintegrated wave form with the deflecting wave form, and means forcombining the resultant of the aforesaid combination of waves with thepicture signals in a phase relationship so as to substantially erasespurious oscillation waves contained in the pictude signals.

ERIC LAWRENCE CASLING WHITE.

